1. Field of the Invention
The present invention relates generally to methods and apparatus for heating of liquids in process tanks used primarily in industrial manufacturing. More particularly the present invention concerns galvanizing systems, particularly corrosive liquids in pickle tanks in which objects to be galvanized are cleaned and prepared for receiving a protective coating of zinc during subsequent galvanizing processes. Even more particularly, the present invention is directed to a heating system for efficiently heating the corrosive, i.e., acidic bath liquid of a pickle tank to enable low cost preparation of the parts for quality galvanizing.
2. Description of the Prior Art
The Galvanizing Process—The hot dip galvanizing process is a method of metallurgically alloying zinc to the outer surface of steel for the purpose of corrosion protection. The process involves processing the metal, typically steel, through several galvanizing process steps as follows:
1. Rack-up—Material to be galvanized is placed in tubs, racks or other fixtures or containers to facilitate transportation of the material through several process tanks.
2. Caustic—The material, also referred to as parts or pieces, is dipped into a bath of liquid caustic cleaner to remove oil, paint, grease, etc. The tank or bath is typically heated to at least 160° F.
3. Rinse—Residual caustic composition is removed in a water rinse bath.
4. Pickling—The material is dipped into an acid solution to remove rust, oxides, and mill scale. The most common acids for use in pickling tanks are hydrochloric acid and sulfuric acid, both being highly corrosive.
5. Rinse—Residual acid is removed in water rinse bath.
6. Pre-flux—The materials are dipped into a solution of zinc ammonium chloride to coat the material prior to transporting the material through a molten zinc bath. The zinc ammonium chloride solution will prevent the materials from oxidizing while the materials are waiting to be passed through the zinc bath.
7. Zinc Bath—The preflux coated material is then dipped into a bath of molten zinc. This is the part of the process where galvanizing of the material takes place. It is also there part of the process where a quantity of ash is created. The molten zinc bath is typically maintained at a temperature of about 840° F., although temperature ranges of the zinc bath may vary form about 825° F. to about 860° F.
In the hot dip galvanizing industry there exists a need to heat a pickle bath of corrosive acidic liquid in process tanks or baths. Currently there are several methods typically used to heat the process tanks. Most commonly is the use of a boiler to heat water to steam and then transport the steam to a bath in pipes, where the steam is either pumped directly into the tank or pumped through heat exchangers to liberate the heat of the steam into the liquid of the bath, thus heating the liquid. This method is quite inefficient and requires special permits to operate. Other methods include hot water heaters which are similar to the boilers, electric and submerged combustion. Electric heating is relatively efficient for heat transfer, but the cost of electricity for maintaining a desired temperature range in a large acidic bath is usually prohibitive in a large galvanizing operation.